This invention relates to polyimide resins, a class of organic compounds well known for their outstanding thermal-oxidative stability.
Polyimides are generally prepared either through direct condensation reactions to afford linear, long chain, polyimides, or through addition reactions preformed on end-capped imide oligomers to give crosslinked polyimides. In both cases, it is well known that high stability is conferred by the use of aromatic or heteroaromatic moieties, while the use of aliphatic moieties reduces thermal-oxidative stability.
Condensation type polyimides have the highest thermal-oxidative stability because aliphatic end caps (which are used in addition type polyimides) are not used. Condensation type polyimides are typically prepared by treating an aryl dianhydride with an aryl diamine in an aprotic solvent to produce a polyamide acid, which is subsequently dehydrated to give the final linear polyimide. Substantial processing problems can arise during these manipulations. For example, the polyamide acid is prone to solvolytic degradation and thus its shelf life can be only a few hours at ambient temperature. Therefore, the polyamide acid requires special handling and storage under refrigeration, which creates inconveniences that greatly increase the ultimate cost to the user.
The dehydration step also creates processing difficulties. Dehydration can require prolonged heating under a vacuum to ensure complete removal of the aprotic solvent used in polyamide acid synthesis and the by-product, water. Alternatively, a chemical dehydration, step can be used, but this requires use of large quantities of strong reagents such as mixture of acetic anhydride and pyridine. Isolation of the polyimide from the reaction mixture and solvent removal require laborious filtration and drying.
Other problems arise with condensation polyimides in one of their principal uses, namely composite materials comprising (i) the polyimide as a matrix material and (ii) fiber reinforcement. The reinforcement of condensation polyimides with fibers poses operational difficulties. High molecular weight polyimides have poor melt flow and must be processed under pressures of between 2000 to 6000 psi. Costs associated with fabricating parts under these conditions are prohibitive for most applications.
Alternatively, polyamide acid has been used to impregnate fibers, and the prepregs thus formed cured thermally to afford polyimide composites. However, because solvents and water are generated during curing, the fabricated parts exhibit excessive void contents resulting in inferior mechanical properties and reduced thermal-oxidative stability.
Processing problems associated with condensation type polyimides have been overcome through the use of addition polymerization type polyimides. Addition polyimides are commonly used in composite manufacturing because an intermediate oligomer (prepolymer) is formed, in situ, and the in situ-formed oligomers are easier to handle than the final polyimide. Furthermore, because no undesirable volatile materials are evolved during the final stage of polymerization, void-free parts can be fabricated. Addition type polyimide resins that are widely used for high temperature polymer matrix composites are typically made according to U.S. Pat. No. 3,745,149, which is incorporated herein by reference. This patent discloses a method for preparing polyimides from mixtures of (a) an dialkyl ester of an aromatic tetracarboxylic acid, (b) an aromatic diamine, and (c) an monoalkyl ester of a dicarboxylic acid (as an end cap) in a molar ratio of n:(n+1):2. The monomers are mixed in an alcoholic solvent, reacted at elevated temperatures to form, in situ, imide oligomers having end caps at both ends, and cured at high temperatures to yield macromolecular polyimides. Polyimide resins prepared by this method are conventionally referred to as PMR resins. Polyimide formulations prepared according to the method of U.S. Pat. No. 3,745,149, when reinforced with various fibers, have sufficient thermal stability and mechanical properties to serve as structural materials during continuous exposure at 550.degree. F. These composites have successfully replaced metals as structural materials in applications such as jet engine cowls and ducts.
However, these addition type polyimides have two significant limitations. First, they are difficult to process. The selection of optimal formulations is restricted by the capability of standard composite fabrication equipment known as the autoclave. Autoclaves are typically constructed to withstand a maximal pressure of 200 to 300 psi only. It has been found that the prepolymers prepared under this patent cannot have a formulated molecular weight of much greater than 1500, beyond which void-free parts cannot be readily autoclaved. Second, such formulations have an aliphatic content of 12% or greater because of the presence of the aliphatic end cap groups. The high aliphatic content can result in severe degradation at higher temperature, thus limiting the high temperature applicability of PMR polyimide resins.
Much interest exists in developing addition type polyimides that can be used at 600 to 800.degree. F. However, with the current technology, better thermal-oxidative stability cannot be accomplished without compromising processability. The adoption of PMR polyimides to a higher temperature range is therefore not economically feasible at present.
In particular, attempts to prepare polyimides that can be used at temperatures above 550.degree. F. according to the methodology described in U.S. Pat. No. 3,745,149 have been unsuccessful. See, for instance, R. Vanucci, SAMPE Quarterly, 19, (1), 31 (1987) and references cited therein. Typically, attempts have been made to synthesize higher molecular weight (about 3000) imide oligomers to minimize the contents of the aliphatic end-caps and thereby improve the thermal-oxidative stability. However, as the molecular weights of the imide oligomers increase, the resin flow decreases. Polyimides formulated according to U.S. Pat. No. 3,745,149 and having molecular weights high enough to withstand long term exposure at 700.degree. F. in air have been found to require pressures of between 2000 and 5000 psi for proper consolidation. The processing of conventional high molecular weight imide oligomers thus can not be conducted with autoclaves, a hurdle that renders polyimides prepared according to U.S. Pat. No. 3,745,149 impractical for use at temperatures substantially above 550.degree. F.
Accordingly, there is a need for compositions of matter and methods for preparing addition type polyimide resins, where the resins are easily prepared, composites containing the resins can be prepared using conventional autoclave equipment, and the resins have the capability of extended operation at elevated temperatures of at least 600.degree. F.